1. Field of the Invention
The present invention relates to a magnetic toner to be used for visualizing an electrostatic charge image in an image forming method for an electrophotograph or the like.
2. Description of the Related Art
In recent years, from a technical viewpoint, an image forming apparatus has been further requested to have a high speed and long-term high reliability in addition to high definition, high appearance quality, and high image quality. A reduction in particle size of toner and sharpening of a particle size distribution have been attempted to achieve a high-resolution and high-definition development mode. However, when the particle size of toner is merely reduced, dispersibility between a binder resin and another internal additive of a magnetic body reduces, so toner performance is apt to be influenced by the reduction. In particular, the influence is remarkable upon high-speed treatment or after long-term use.
In particular, in the case of magnetic toner used for a one-component development mode in which a reduction in size of an apparatus is advantageous, the dispersed state of a magnetic body in the toner may cause a problem such as the fluctuation or deterioration of anyone of various properties requested for magnetic toner such as development property and durability.
When magnetic body particles are insufficiently dispersed into magnetic toner particles, the total amount of magnetic body particles exposed to the toner particle surfaces is changed by individual magnetic toner particles. When the amount of magnetic body particles on the toner particle surfaces is small, the toner particle surfaces have high charge amounts when they are subjected to triboelectric charging with a charge imparting member (developing sleeve), so charge-up occurs. On the other hand, when the amount of magnetic body particles on the toner particles is excessively large, charge is apt to leak, so a high charge amount is hardly obtained. Moreover, toner opposite in polarity is apt to generate owing to contact between any one of the magnetic body particles and a binder resin, so the width of a charge distribution expands. The expansion may be responsible for the deterioration of image quality. For example, fine-line reproducibility is apt to reduce, or image roughness is remarkable, so it becomes difficult to cope with a recent demand for high image quality.
Magnetic toner contains a magnetic body to provide magnetism, so the magnetic force of the toner causes a toner coat layer on a magnetic toner bearing member (developing sleeve) to form the napping of magnetism. In jumping development using magnetic toner, an image is generally developed from above a magnetic toner bearing member to a photosensitive drum through the application of a developing bias while a nap shape is maintained to some extent.
When magnetic body particles are insufficiently dispersed into magnetic toner particles and a variation in magnetic properties of toner particles is excessively wide, napping is apt to be disturbed. When the napping is disturbed (the napping is excessively long, excessively thick, or is nonuniform in size), for example, a problem in which the napping scatters to the periphery of an image or a problem in which fogging in which a non-image portion is developed with toner is apt to be remarkable occurs.
In addition, when napping is excessively long or excessively thick, a toner mounting height on a photosensitive member increases, so the tailing of a fixed image due to thermocompression fixing is apt to occur. In addition, when the napping shape of magnetism remains even on transfer residual toner, a flaw tends to occur on the photosensitive member owing to rubbing with a cleaning blade.
In addition, such expansion of the width of a charge distribution due to insufficient dispersion of a magnetic body as described above is apt to cause so-called selective development in which toner having a certain range of charge amount distribution is preferentially consumed. At the same time, the progress of the selective development may further accelerate the above problems.
In particular, in order to cope with recent trends toward a high speed and a long lifetime, a large-capacity process cartridge with an increased process speed and an increased toner loading weight in a developing unit has been used. However, the use of such process cartridge tends to make the above problems more remarkable, so quick alleviation of such state has been desired.
Meanwhile, when development conditions are set in such a manner that an image density is sufficiently high (for example, the amplitude of the alternating component of a developing bias is increased), particularly in the case where napping is disturbed, excessive toner is apt to be used for development, so the toner mounting amount of an image increases. As a result, image quality is apt to deteriorate, fogging is apt to be remarkable, or a toner consumption is apt to increase.
When a developing unit is set in such a manner that a toner consumption reduces (for example, the amplitude of the alternating component of a developing bias is reduced), an image density tends to reduce or a line width tends to be small. Therefore, the control of the performance of magnetic toner, in particular, the control of napping due to a magnetic body to be incorporated into the toner is more important than the setting of development conditions for achieving high image quality while maintaining a high image density and a low toner consumption.
With regard to a magnetic body to be incorporated into magnetic toner, each of JP 09-59024 A and JP 09-59025 A has conventionally described magnetite particles each containing 1.7 to 4.5 atom % of Si and less than 10 atom % of one or two or more metal elements selected from the group consisting of Mn, Zn, Ni, Cu, Al, and Ti as a metal element except iron in terms of Si with respect to Fe. The magnetite particles improve magnetic properties and chargeability. However, merely adding the above metals has been still unable to reduce a toner consumption, so the particles are susceptible to improvement.
In addition, JP 04-184354 A, JP 04-223487 A, and the like each disclose a method of reducing the saturation magnetization of toner involving, for example, replacing ferrous of magnetite with a divalent metal such as zinc or copper. However, the method involves the emergence of a problem such as an increase in fogging in a development method using an alternating electric field particularly at a low temperature and a low humidity, so the method is not sufficient for the achievement of the stabilization of image quality or a reduction in consumption.
In addition, each of JP 2003-98731 A, JP 2003-107792 A, and JP2002-372801 A discloses toner causing no image contamination and excellent in fine-line reproducibility while maintaining good chargeability through the control of magnetization in a magnetic field of 5 kOe or 1 kOe. The use of such toner for a two-component developer does exert an excellent effect. However, the magnetization of the toner is so low that the toner cannot be used for a one-component developer. Therefore, the toner has been still unable to alleviate reductions in image quality and developability in long-term use particularly in a high-speed, large-capacity cartridge sufficiently, to reduce a toner consumption sufficiently, and to alleviate the tailing of a fixed image sufficiently, so the toner is susceptible to improvement.
Each of JP 07-301948 A and JP 07-333889 A describes magnetic toner with which a short nap can be formed and a high-quality image can be obtained by adjusting a saturation magnetization amount in a magnetic field of 1 kOe and a value for the product of the weight average particle size and density of the toner. However, napping may be disturbed after the performance of a long-term durability test. As a result, for example, the tailing of a fixed image is apt to occur, fine-line reproducibility is apt to reduce, or a toner consumption is apt to increase. Therefore, the toner must be improved before it is applied to a high-speed machine.
Meanwhile, each of JP 03-101743 A and JP 03-101744 A describes that the particle sizes of magnetic body particles are reduced and a particle size distribution is narrowed for uniformly dispersing the magnetic body particles into toner particles. Those measures surely tend to uniformize the dispersion of the magnetic body particles into the toner particles. However, when the particle size of toner is reduced for achieving high image quality, fogging is accelerated. Therefore, the dispersibility of magnetic body particles into toner particles is susceptible to improvement.
As described above, at present, the realization of magnetic toner which is excellent in durability and developability even when it is applied to a high-speed developing system having a high process speed and using a large-capacity cartridge, which can provide an image with a sufficient image density and high image quality when it is used in a small amount, and which suppresses the tailing of a fixed image and the occurrence of a photosensitive member flaw requires further investigation.